In the automotive vehicle industry today, it is necessary to comply with government regulations that establish certain minimum strength requirements for the side doors of the vehicles. The purpose of such regulations is to minimize the safety hazard caused by an intrusion into the passenger compartment in a side impact accident.
Various types of reinforcing door beams have been proposed to meet these requirements, e.g., tubular steel beams, sheet steel stampings, and roll formed high strength steel sections of various configurations. Engineers responsible for designing reinforcing door beams must contend with a number of requirements that often conflict with one another. For example, it is desirable to maximize the performance of reinforcing door beams by designing beams that absorb high loads (principally bending loads), deflect significantly before failure, and absorb as much energy as possible during the absorption of impact loads. At the same time, it is also desirable to minimize the weight and size of reinforcing door beams. With regard to size, placement of a door reinforcing beam within an automobile door assembly makes it important for the door reinforcing beam to have a relatively small cross-sectional geometry, in order to avoid interference with other structures and/or mechanisms within the door, such as those used to operate a window associated with the door. Such size and weight considerations can make it difficult to achieve the desired performance of reinforcing door beams for load capacity, deflection before failure, and impact energy absorption.
Previously proposed door beams provided the desired degree of high strength, but often presented other disadvantages. For example, martensitic steel has limited ductility, thus placing some restrictions on the permissible cross sectional configurations obtainable by roll forming. Accordingly, past designs for door reinforcement members constructed of high strength martensitic steel typically had cross-sectional geometries, such as, for example, a hat-shaped cross-sectional geometry, that buckled or spread on bending, thereby reducing mass effectiveness in providing side impact protection.